Intestinal microbial flora normally co-exist in a delicate balance with the host, with intestinal epithelial cells providing a physical barrier to prevent access of microbes to deeper tissues and a tightly regulated cellular immune system in the underlying lamina propria (LP) providing protection against invading pathogens or commensals that breach the epithelial barrier. Intestinal dendritic cells (IDCs), potent antigen presenting cells (APC) that bridge innate and adaptive immunity, are able to sample luminal bacteria and likely play a key role in mediating tolerance to commensals as well as defense against pathogens. HIV-1 has been shown to replicate in gut-associated lymphoid tissue (GALT) in the acute stages of infection, resulting in an early, massive, and persistent depletion of CCR5+CD4+ T cells, and in particular of IL-17-producing CD4+ T cells (Th17 cells). Th17 cells have been implicated as mediators of intestinal inflammation but are also proposed to play a role in normal mucosal defense and homeostasis by limiting commensal bacterial penetration. Translocation of bacterial products and DNA into the systemic circulation has been described in chronic HIV-1 infection, but whether this process impacts HIV-1 replication or pathogenesis in the intestinal mucosa is unknown. We have identified significant frequencies of human LP effector CD4+ T cells that produce IFN-3 and IL-2 (Th1) or IL-17 (Th17) in response to commensal bacteria in normal human intestinal tissue. Importantly, bacteria-specific LP CD4+ T cell proliferation in vitro was dependent on the presence of CD1c+ LP DCs. This subset of LP DCs was shown to produce the inflammatory, Th17-biasing cytokine IL-23 upon stimulation with ligands for Toll-like receptors (TLRs) 7/8 that simulate HIV-1 ssRNA. Furthermore, we show that addition of commensal bacteria to HIV-1-infected lamina propria mononuclear cell (LPMC) cultures resulted in increased infection of LP CD4+ T cells. We hypothesize that HIV-1 replication in the intestinal mucosa disrupts intestinal homeostasis through interactions with resident IDCs by skewing the local host response to commensal bacteria away from regulation and toward inflammation. In this proposal, we will address the pathogenic mechanisms whereby HIV-1 disrupts the normal process of intestinal homeostasis and bacterial defense. Specifically, we propose to 1) investigate the mechanisms by which HIV-1 alters human innate and adaptive responses to commensal bacteria in vitro, 2) define the mechanisms by which commensal bacteria influence HIV-1 replication in intestinal mucosa in vitro, and 3) evaluate the relationship between mucosal immune function and bacterial species diversity in intestinal mucosa and plasma samples from a cohort of untreated, HIV-1- infected subjects. We believe that the knowledge gained from these studies will contribute significantly to the understanding of HIV-1 pathogenic mechanisms and facilitate the development of rational therapies for HIV-1 that decrease chronic inflammation and restore intestinal immunity.